Change "defdynamic" to "defndynamic" in guide
Change "defdynamic" to "defndynamic" in guide

Based on the docs, I expected that `defdynamic` would allow me to define and use a function at the REPL. Instead I got the following error:

```
> (defdynamic square [x]
     (* x x))
Invalid args to `defdynamic`: (defdynamic square [x] (* x x)) (did you try to define a dynamic function - use 'defndynamic' instead) at REPL:1:1.
```

Definitely props on the helpfulness of the error message, which makes this an easily overcome issue. However, I'd like to propose this change to the guide so that any other newcomers don't hit a bump the first time they try something.
carp-lang/Carp,carp-lang/Carp,eriksvedang/Carp,eriksvedang/Carp,carp-lang/Carp